sd_is_fifo() may be called to check
whether the specified file descriptor refers to a FIFO or pipe. If
the path parameter is not
NULL, it is checked whether the FIFO is bound
to the specified file system path.

sd_is_socket() may be called to check
whether the specified file descriptor refers to a socket. If the
family parameter is not
AF_UNSPEC, it is checked whether the socket
is of the specified family (AF_UNIX, AF_INET,
...). If the type parameter is not 0, it is
checked whether the socket is of the specified type
(SOCK_STREAM,
SOCK_DGRAM, ...). If the
listening parameter is positive, it is
checked whether the socket is in accepting mode, i.e.
listen() has been called for it. If
listening is 0, it is checked whether the
socket is not in this mode. If the parameter is negative, no such
check is made. The listening parameter
should only be used for stream sockets and should be set to a
negative value otherwise.

sd_is_socket_inet() is similar to
sd_is_socket(), but optionally checks the
IPv4 or IPv6 port number the socket is bound to, unless
port is zero. For this call
family must be passed as either
AF_UNSPEC, AF_INET, or
AF_INET6.

sd_is_socket_unix() is similar to
sd_is_socket() but optionally checks the
AF_UNIX path the socket is bound to, unless
the path parameter is
NULL. For normal file system
AF_UNIX sockets, set the
length parameter to 0. For Linux abstract
namespace sockets, set the length to the
size of the address, including the initial 0 byte, and set the
path to the initial 0 byte of the socket
address.

sd_is_mq() may be called to check
whether the specified file descriptor refers to a POSIX message
queue. If the path parameter is not
NULL, it is checked whether the message queue
is bound to the specified name.

sd_is_special() may be called to check
whether the specified file descriptor refers to a special file. If
the path parameter is not
NULL, it is checked whether the file
descriptor is bound to the specified file name. Special files in
this context are character device nodes and files in
/proc or /sys.

On failure, these calls return a negative errno-style error
code. If the file descriptor is of the specified type and bound to
the specified address, a positive return value is returned,
otherwise zero.